Double clicking to do the 'edge ring' or 'edge loop' selection in Blender 2.8
Enable "Emulate 3 Button Mouse" in Preferences > Input to get double-click loop selection back. There's a discussion on the dev forum about it. Basically, double-click broke two features – Selection Cycling and Select All Boundaries – according to the developers, which is why they disabled it without having 3 Button Mouse emulation enabled.